Minor fix in safe_print function

Minor fix in safe_print function

This commit fixes two issues reported by Coverity:

- Fixes the check whether the buffer is full in safe_print_hex
- Removes dead code in safe_print for floating-point values

Additionally, it fixes an issue reported by Andy where the names of the
statistics were printed as "<unsupported>" due to calling the const char*
version instead of the std::string version of safe_print.

Finally, this fixes an issue where --segv-spin would not print the program name
because it was a const char*. The program name is now stored as a string.

NOTE: As a side effect, the last part also fixes Coverity issue 1362944, which
has been in CVC4 for a long time.